NO337280B1 - Improvement in air-cooled heat exchangers - Google Patents

Description

Technical field

[0001] The present invention relates to improvements in connection with air-cooled heat exchangers, and more specifically, heat exchangers arranged on liquid process plants. Most specifically, the invention relates to improvements in connection with air-cooled heat exchangers on LNG process plants which are

arranged on offshore floats.

Background

[0002] Natural gas is becoming more important as the world's energy needs increase. Natural gas is readily available, especially with the new technologies now being used to extract and use shale gas. Natural gas is much cleaner burning than oil and coal, and does not have the danger or waste disposal problems associated with nuclear power. Post-combustion emissions of greenhouse gases from natural gas are lower than for oil, and only about a third of the emissions from coal.

[0003] There is considerable international trade in natural gas, and the price varies considerably in different parts of the world. A large part of this trade is in the form of liquefied natural gas (LNG). LNG is produced using two main processing steps. The first step is gas pretreatment to remove components that can solidify when cooled to cryogenic temperatures, mainly acidic components and water. Trace elements, mainly mercury which can form amalgam - especially with aluminum process components - are also removed from the gas.

[0004] Heavy hydrocarbon fractions or condensate (NGL) can be removed from the gas in the first or second of the two LNG treatment stages. The second treatment step is mainly condensation of the purified gas, which consists mainly of methane. The methane, together with small amounts of heavier components, is liquefied at atmospheric pressure and around -163° C. After liquefaction, LNG is transported to its destination and converted into gas.

[0005] Processing of natural gas to produce LNG has traditionally been done at large land-based facilities, which include the two steps of pretreatment and LNG at the same site. The latest developments in technology and markets have enabled the construction of LNG plants on floating structures, a development that has inspired the relocation of a significant part of LNG processing plants offshore to liquefied natural gas condensing plants (FLNG).

[0006] The FLNGs are usually designed to be located at a distance from a coast and are connected to natural gas by pipelines. The FLNGs are typically also designed to act as buffer storage for LNG and as terminals for loading LNG tankers used to transport LNG to the markets.

[0007] The latest development towards FLNGs has made offshore natural gas resources more accessible to the market, and has resulted in a reduction of capital costs for establishing an LNG plant. Other important drivers for offshore LNG include reduction of land-based environmental impacts; reduction of land use problems for equipment and infrastructure; and reduced likelihood of resistance from local communities. The entire FLNG plant can be built at a shipyard, which is efficient and improves quality control, cost control and reduces construction time. FLNGs are also mobile and can be moved to alternative locations if necessary.

[0008] Many studies of FLNG technologies have been carried out during the last couple of decades. Currently, several projects are underway worldwide. To date, actual construction has started for three units: the Shell Prelude project, the Exmar / Pacific Rubicales barge project, and the Petronas FLNG 1 project.

[0009] In these and other projects, both gas pre-processing and condensation will typically be located on top of the FLNG. The space below deck is used for LNG storage and marine-specific equipment. The area available on an FLNG deck is usually only about 20% of the area used for similar facilities on land. Among other design constraints, this reduced area for the process represents a safety issue, including proximity to residential quarters and limited space for safety barriers. It is also important that it also limits the size of processing plants and the possibilities for exploiting economies of scale.

[0010] Consequently, in addition to safety issues, the condensation process entails environmental problems. The liquefaction process generates large amounts of heat, which must be transferred to the environment. With the current design, large quantities of seawater are needed for cooling purposes on board the FLNG, water which is then released back into the sea at a higher temperature. This can be harmful to marine life, as due to mechanical stresses in seawater pipes, pumps and other equipment, toxic chemicals are used to prevent fouling, and the increased water temperature. In coastal waters, where marine life is abundant, certain jurisdictions do not allow the use of seawater for cooling at all, with others expected to follow suit.

[0011] The alternative to seawater cooling is air cooling. However, air cooling requires significantly more space than seawater cooling. This space is proportional to the cooling demand and the Logarithmic Average Temperature Difference (LMTD) between the air and the process liquid to be cooled. The footprint of a well-designed air cooling system, typically with an LMTD of 30<0>C, can be in the order of 1000 m<2>per 100MW of cooling demand. This is a challenge, especially when the available space is only about 20% of the space used on land for similar facilities.

[0012] Condensing plants are usually either efficient base load systems or less efficient but simpler peak-shaving systems. Known refrigerants, such as hydrocarbons or nitrogen, circulate in refrigeration systems that include compressors, air-cooled heat exchangers, and LNG exchangers. Depending on the cooling system as refrigerants, the refrigerant may or may not be condensed in the air coolers before being taken to the LNG exchangers.

[0013] In normal situations, it will be desirable or necessary to cool / condense the refrigerants to about 30° to 40° C before the refrigerant is fed to the LNG exchangers. However, in temperate regions, the design ambient air temperature may be relatively high (for example, 32 °C (90 °F)) or higher, and it is expected that the approach temperature for the air-cooled heat exchanger should be at least 10 °C, preferably 15 ° C or more.

[0014] Engineers in the field will know that this problem can be solved by operating the compressor interstage coolers at higher temperatures, and compressing the refrigerants, especially a refrigerant to be condensed, to higher pressures than normal. All cooling and condensation therefore takes place at higher temperatures, which enables air-cooling with high LMTD and high air cooler approach temperatures even in temperate regions. Unfortunately, all this significantly reduces the condensing efficiency, increases the energy demand and thus increases the cooling demand, which partially counteracts the strategy of increasing the LMTD and achieving air cooling.

[0015] Table 1, see below, illustrates this by comparing work and cooling requirements for two liquefaction processes with water and air cooling. The liquefaction rate is 400 metric tons per hour, the feed gas is at 60 bar and 25 °C, and consists of 98 mol% methane, 1.5 mol% ethane and 0.5 mol% propane

[0016] As can be seen from table 1, the cooling demand for air-cooled systems is

significantly higher than the cooling requirement for a water-cooled system.

[0017] Air cooling as such is a well-known technology, and is widely used on land for power plants, buildings and many other purposes. Figure 1 illustrates an air cooler 1 arranged on a stand 2. Incoming air for cooling flows into

the air cooler from below as indicated by arrows 3, and outgoing air, and heated, air is discharged at the top of the cooler as shown by arrows 4. heat transfer medium circulation is indicated by lines 5. The air

is sucked in from the bottom, using a fan. This air passes over a spiral, which contains the process liquid to be cooled. The air that flows out of the air cooler is normally warmer than the surrounding air, and will tend to rise due to the lower density. However, part of the outgoing heated air can flow back into the air intake and thus reduce the cooling efficiency.

[0018] On a float, with different wind directions and large arrays of air coolers, such recirculation would be likely, and would be detrimental to the performance of the air cooling system. On land, this problem is partially solved by spreading air coolers over a large area, and by providing a high air velocity out of the coolers.

[0019] Air coolers are also susceptible to fouling or deposition of contaminants on heat exchanger device surfaces such as tubes or finned tubes containing heat transfer medium. On floats, such contaminants can be salt, smoke or oil mist. This reduces the heat transfer efficiency. In many situations, such fouling is predicted in advance, and coolers are oversized accordingly. With limited space on a float, such oversizing may not be practical. Gas turbines in coastal or offshore areas are known to have similar problems. In this case, the solution has been to fit intake air filters.

[0020] A further challenge is determining the design air temperature. In many areas, the annual average air temperature is much lower than the summer peak temperature. Furthermore, the summer peak temperature can occur only a few days a year. It can be highly desirable to size air coolers based on an average temperature instead of the peak temperature, as this can reduce the number of air coolers.

[0021] However, this means that on hot days the cooling capacity can be greatly reduced. In some cases, this problem is solved using a design air temperature that is lower than the annual peak temperature, and using a water shower at the air cooler intake on very hot days. This reduces the temperature to acceptable levels, because the air's wet temperature is usually lower than the dry air's temperature.

[0022] The most important factor that determines the economics of a FLNG is the LNG production rate. Higher production rate requires proportionately more cooling and increases the air cooler footprint accordingly.

[0023] All this shows that it is very desirable to have as much air cooling capacity as

possible, especially at FLNGs where space is limited.

[0024] A recent adaptation of FLNG, which increases the space available on a floater, is a coastal liquefaction, storage and offloading (CLSO) device. The CLSO fit addresses FLNG space limitations, safety, environmental concerns and most importantly, processing capacity. The first treatment step, gas pre-processing, is mainly carried out on land, at separate terminals or on dedicated floating systems, instead of taking up valuable space on FLNG. The pre-processed gas is then fed to one or more floating CLSOs, which now have much more deck space available. Extra deck space on CLSO, freed up by removing pre-processing, can be used for extra security. The extra deck space also opens up the possibility of using air cooling instead of seawater cooling, which solves the problems of seawater intake and associated environmental problems. However, it would be better to use this space for additional production capacity, without falling back on seawater cooling, if possible. Furthermore, there are opportunities for higher liquefaction capacity with resulting economic benefits.

[0025] A large CLSO can e.g. have a length of 350 m and a width of 60 m, corresponding to the hull dimensions of existing, successful vessel designs. The deck space is therefore around 20,000 m<2>. Desired production may be in the order of 1,000 tonnes of LNG per hour for this size CLSO. According to table 1, for an air-cooled base load system, this will require at least 600 MW of cooling. With air coolers requiring around 1,000 m2 per 100 MW of cooling, this would require 6,000 m<2>deck area or about a third of the CLSO deck.

[0026] Given these requirements, air coolers placed on deck entail certain problems in terms of design and economy: it will be very difficult to ensure that all coolers receive fresh air and not air that is partially recycled and that it therefore becomes too hot for efficient operation ; for such a large range of coolers, provision of intake air filters and water diffusers on hot days would become unwieldy; and since LNG production capacity is dependent on available space air coolers, if placed directly on deck, will significantly reduce available space for LNG capacity.

[0027] An alternative design is to locate air coolers on an outrigger 6 arranged on one side of the hull 7 as shown in figure 2. For a float of the size mentioned above, the bracket has to be approximately 17 m wide in its entire length of floater, e.g. approximately 350 m, to provide an area of 6,000 m2. In addition to this, areas for air cooler access ways may be required for maintenance. This is not very practical. In addition, air coolers will be exposed to salt seawater mist. The need to provide filters and fresh water spray on the coolers on hot days will further complicate this design approach.

[0028] Alternative configurations are shown in figures 3 and 4. These configurations raise other problems, such as height, hot air being blown into the tire by the float or non-symmetric torque created by the airflow from the coolers. Air recirculation, which is detrimental to the air cooling efficiency, would be a problem in all these cases.

[0029] From GB903397A it is known an air cooler assembly comprising a plurality of air coolers where the air coolers are arranged in two parallel rows next to each other on a mainly horizontal plate with side walls where the horizontal plate and the side walls form an intake chamber, which draws in air in large longitudinal openings in the side walls.

[0030] N0328852B, describes a floating of two converted LNG tankers to provide a catamaran hull. Some of the LNG storage tanks have been removed to make room for LNG processing equipment. Water is used here for cooling for the production of LNG.

[0031] One purpose of the present invention is therefore to provide

improvements in air cooling efficiency for such floats as described above. Summary of the invention

[0032] According to a first aspect, the present invention relates to an air cooler assembly comprising a plurality of air coolers, characterized in that the air coolers are arranged on a channel, the channel has the form of a straight prism which has a polygonal cross-section, and where the channel has one air intake to take in cooling air to be distributed to all air coolers. Arrangement of air coolers with a channel that has an air inlet, and where the air introduced through the air inlet is distributed until all air coolers are arranged on the pipe, makes it possible to control the intake of air and to determine that air heated by cooling air coolers is not recycled back to the air coolers to reduce their cooling efficiency. In addition, controlled intake of air for cooling allows the removal of seawater, solids etc. from the cooling air to avoid the deposition of salt and other solids on the heat exchanger surfaces, which can result in reduced efficiency of these.

Consequently, maintenance for the removal of solids from

the heat exchange surfaces will be significantly reduced.

[0033] According to one embodiment, the longitudinal axis of the channel is essentially arranged horizontally. The channel is preferably aligned substantially horizontally to control the height of the structure. For use on board floats, as described here, tall structures must be kept to a minimum to avoid instability in the floating structure and to keep the wind vane effect low.

[0034] According to one embodiment, the air inlet is provided with one or more fan(s). Fans in the air intake are preferred to ensure that air is introduced into the duct via the air intake, only. The fan(s) create a certain excess pressure inside the duct in relation to the surroundings, to avoid uncontrolled ingress of air through other openings in the duct. In addition, the fans are important to overcome the pressure drop across any equipment for the removal of water and/or solids from the incoming air.

[0035] The air inlet can be provided with separators for separating liquid and particles from the incoming air. The content of liquids, such as salt water, and solids in the incoming air must be kept low to prevent salt and other solids from depositing or settling on the heat exchanger surfaces of air coolers. The separators are a means of said reduction.

[0036] According to one embodiment, the air inlet is provided with one or more spray nozzle(s) for moistening and cooling incoming air. Controlled introduction of water in the form of a spray may be preferred to increase the humidity in the air, and thus the heat capacity of this to increase the efficiency of the air cooler.

[0037] A filter for removing small water droplets can be arranged downstream of

spray nozzle(s).

[0038] According to another aspect of the present invention, a floater for LNG production which comprises a plurality of air coolers to achieve the necessary cooling capacity, characterized in that the air cooler is arranged on a channel, where the channel has the form of a straight prism with a polygonal cross-section, and where the channel has one air intake to take in cooling air to be distributed to all air coolers.

[0039] According to one embodiment, the float is anchored via a swivel mooring or a tower, where the float is allowed to turn according to the weather to keep the air inlet against the wind. By holding the float in place by means of an anchored turret column, where a bearing device enables the float to rotate according to the weather by moving around the turret column, and a fluid transfer system comprising a swivel with associated equipment, where the float element and duct air inlet can be held mainly upwind. By keeping the floating element and thus the air inlet against the wind, it is ensured that the coldest air that is available is led into the air intake, and that the air that is heated in the air coolers and released into the surroundings, is not returned into the duct and finally the air coolers, as this would result in warmer air in the duct and thus a lower cooling effect.

[0040] According to one embodiment, the float is an elongated ship-shaped float that has a bow end and a stern, where the tower and the swivel are arranged at the bow end of the float element and where the channel is arranged essentially parallel to the longitudinal axis of the float.

[0041] According to a third aspect, the present invention relates to a method for producing LNG from natural gas on board a float, which is characterized by the following steps:

- Bringing pre-treated natural gas on board a float,

- Cooling of the natural gas to produce LNG by repeated cycles consisting of compression, cooling and expansion of a refrigerant and heat exchange between the cold refrigerant and the natural gas to cool the natural gas,

The characteristic that cooling of the refrigerant during the production of LNG is carried out using air coolers is arranged on one or more channels which are arranged so that all air coolers receive air from the inside of said one or more channel(s), and where the air used in air coolers is released to the environment through the air cooler(s).

[0042] According to one embodiment, the channel(s) are oriented so that the air inlet thereof is against the wind in relation to the air coolers being arranged on the pipe.

Brief description of drawings

[0043] Figure 1 shows a traditional arrangement of an air cooler stand, and the air flow into and out of the air cooler; Figure 2 illustrates air coolers arranged on a horizontally arranged cantilever; Figure 3 shows air coolers arranged on an inclined cantilever; Figure 4 shows an alternative cantilever having both horizontal and inclined areas for the arrangement of coolers; Figure 5 illustrates movement of a turret-anchored vessel or float in response to wind; Figure 6 is a cross-section through a cooler channel according to the present invention; Figure 7 is a cross-section of an alternative cooler channel according to the present invention; Figure 8 is a side view of the cooler channel according to the present invention arranged on a float; and Figure 9 is a bird's eye view of coolers and floats as shown in Figure 8. Detailed description of the invention

[0044] According to the present invention, LNG plants with production associated with power production will be placed on offshore floats 10 as shown in Figures 5, 8, 9. The floating element is preferably ship-shaped, i.e. designed as an elongated floating "hull" , which has a bow-shaped front end and a rear end. The float 10 is anchored with one or more anchors which are connected to the float via a turret, anchored with anchor lines C, and the associated bearing and turning system 11 arranged at or near the bow of the float. The person skilled in the art will understand that the term "turret" is used to include a turntable anchoring and loading/unloading system that is widely used in offshore operations, and other similar solutions that can be used to anchor a ship so that the vessel can rotate with the wind and which includes pipe connections.

[0045] Preferably, the gas to be cooled and liquefied is pre-treated with one or more of the following processes: - Gas neutralization, i.e. removal of unwanted acid gases from the natural gas, - Dehydration, i.e. removal of water that could otherwise lead to formation of hydrates from gas,

- Hg removal,

- Full or partial NGL treatment, i.e. separation of NGL from the gas and / or reception of NGL separated from the gas on floats, and fractionation of NGL into salable products, typically ethane, propane, butane and the heavier C5 + fraction, and

- Compression of the pre-processed natural gas.

[0046] Pretreatment is performed to reduce the processing on board the floater to only the liquefaction, and to avoid separation of NGLs that must be treated separately.

[0047] The pre-treated natural gas to be liquefied is supplied via a gas pipe 9 (see figure 8) connected to the turret and the swivel 11. The float 10 is free to

rotate by force to the wind about the turret, so that the float 10 will turn with the wind as indicated by the arrows 12 by means of the action of the wind indicated at 13, to keep the bow substantially upwind. Not shown thrusters can be provided on the float to adjust the direction of the float in the event that the wind is too light to turn the float to the preferred direction. The expert will also understand that it may be preferable to position the float so that the bow does not point directly upwind, but deviates by an angle of e.g. 5 to 20 degrees from the upwind direction, to allow any gas leaks on deck to blow away from the tire partially sideways.

[0048] LNG liquefied aboard the float can be stored in buffer tanks in the float's hull and loaded for export to non-illustrated LNG tankers fixed to the float to load the LNG, and then transport the LNG to its destination. Normally, such LNG tankers will moor to one side of the float.

[0049] The liquefaction will be carried out using known technologies, either efficient baseload condensation plants or simpler "peak sheaving" LNG technology. Liquification processes are driven by compressors with inter- and after-coolers, where compressors and coolers reduce the enthalpy of the refrigerant(s).

[0050] The low-enthalpy refrigerant(s) is introduced into the LNG exchanger(s) where the pre-treated gas is pre-cooled, liquefied and sub-cooled. The resulting LNG is stable at atmospheric pressure and approximately -163 °C. The refrigerant, which has a higher enthalpy when exiting the LNG exchanger(s), is fed back to the compressor's suction side.

[0051] According to the present invention, air coolers are arranged on one or more air cooler channel(s) 15 arranged along one side of the float. The air cooler duct 15 can have any suitable cross-section. The air cooler channel 15 preferably has a straight, polygonal prism shape, and has an essentially horizontal longitudinal axis. The embodiments shown in Figures 8 and 9 have rectangular or rhombic cross-sections, but the person skilled in the art will understand that different polygonal cross-sections are relevant. The length of the air cooling duct can be equal to the total length of the float, but can be somewhat shorter, such as 2 to 20% shorter than the float. The air duct has an air intake opening 14 at the end closes to the front, or bend, it floats, and an air outlet 21 opening at the end closest to the aft of the floater 10.

[0052] Due to the wind arrangement of the float, the air intake opening 14 will be against the wind, and the air outlet is away from the wind, which ensures that the air released from air coolers is not returned to air coolers. This optimizes the air cooling effect.

[0053] A weather cover 17 is preferably arranged in front of the air intake opening 14 to stop or significantly reduce the penetration of seawater into the air channel 15. A person skilled in the art will understand that the weather cover 17 advantageously comprises a metal grid or a screen arranged so that air is allowed to flow through, but which stops a significant part of the water as a result of the air flow. Downstream of the weather cover 17, separators 18 are arranged to stop water solids, droplets and droplets above a given size. Preferably, droplets and solids larger than 50 microns in diameter are stopped. More preferably, solids and droplets with a size greater than 30, for example 20 microns, are stopped by the weather hood and separators. The separators 18 may be any form of packing known to those skilled in the art to be applicable to said task.

[0054] Spray nozzles 19 for moistening the incoming air can be arranged downstream of the separators 18. The water used for moistening is fresh water or desalinated water to avoid salt deposits on the inside of the channel 15. A filter 20 is preferably arranged downstream of the spray nozzles to remove excess drops from the spray. By injecting water into the air stream, the air can be cooled from a dry temperature to a wet temperature, which can reduce the temperature by, for example, approx. 5 ° C. The water used for spraying should be distilled water, transported to the float on a supply vessel.

[0055] One or more fan(s) 16 are arranged in the air intake 14 for the channel 15 to ensure sufficient airflow into the channel 15. The fan(s) 16 shown are arranged downstream of the filter 20. The fan(s) 16 can alternatively be arranged between the separators 18 and the filter 20.

[0056] Air coolers 1 are arranged on the side walls of the air cooler duct 15, so that air for cooling the coolers 1 is drawn out from the inside of the air cooler duct 15, and released into the surroundings. This arrangement, together with the wind setting of the float, ensures that hot air discharged from air coolers is not recirculated into the same or a neighboring air cooler.

[0057] Lamella 21 is preferably arranged at the aft end of the channel 15, to ensure a small excess pressure inside the channel in relation to the surrounding pressure. By keeping an overpressure inside the channel of, for example, 0.002 bar, air penetrating into the channel, apart from through the air intake 14, can be avoided. The slats 21, or another suitable control means, can be adjusted to maintain such a low excess pressure.

[0058] To be able to use air coolers at all, the liquefaction plant must be adapted so that the coolant in the compressor's intercooler and aftercooler systems is warmer than normal, and must therefore be able to transfer the waste heat to warm ambient air. Professionals know how to do this, but doing so will always increase the specific compressor power and thus the cooling power.

[0059] The increased compressor power and increased cooling power carry large costs in the form of reduced LNG capacity, especially on a CLSO where available power and space are limited. Compared to building a second float, it would be much more cost-effective if the compressor power could be reduced, even when air coolers are used, and to increase the condensing capacity accordingly.

[0060] Compressor power will be reduced if air cooling capacity is increased by increasing the number of air coolers, if fouling of air cooler heat transfer surfaces is reduced, and if recirculation of hot air over air coolers is minimized in all weather conditions. In addition, the air cooling capacity is increased if the air temperature is reduced, for example on hot days, with a water shower, which reduces the temperature from the dry temperature to the lower wet temperature.

[0061] This cooler arrangement significantly increases the available space for air coolers, compared to normal use where the air is released in an upward direction. This cooler arrangement also ensures that the hot air is not redirected into the air coolers, so the cooling air is fed into the duct at the top end. By directing the flow or released hot air upwards and downwards, transversely directed forces that could rotate the float will not be created by the air coolers. The person skilled in the art will understand that the channel 15 can be arranged on the outside of the tire of the floating element as described, or can be arranged above the tire. In addition, more than one channel can be arranged on a float, for example two or more air channels, according to the need for this.

[0062] Depending on the configuration of the channel 15, air coolers 1 can be arranged on two or more of the surfaces of the air channel 15. Figure 7 illustrates an air channel which has a rhombic cross-section, and where longitudinal sections through the opposite corners of the cross-section are respectively vertical and horizontal . The air coolers 1 are arranged on all four side walls of the duct 15 as shown in Figure 7, and the air coolers are arranged symmetrically about horizontal longitudinal sections through the upper and lower corners of the duct, so that laterally directed forces as a result of the action of fans in the air coolers counteract each other , and does not result in transverse forces on the float.

[0063] The resulting prevailing wind direction relative to the float ensures that the hot air leaving the air coolers is blown away from the air intake to the duct 15. As mentioned above, the turret and swivel arrangement 11 on the float element 10 enables the float to pivot the wind so that the bow is essentially pointed upwind, such as straight into the wind or deviation e.g. 2 to 20 ° the direction of the wind. Preferably, the channel and the air coolers are arranged leeward, or the downwind side at the bottom or the windward side of the float if the float has an orientation that deviates from pointing straight into the wind. It may be preferable to arrange the turret, or to construct the float, so that the float does not turn with the wind so that the bow is straight into the wind. A deviation from the prevailing incoming wind direction, e.g. from 5 to 15 degrees from a position directly into the wind, it may be preferred for

to ensure that any gas due to gas leaks on the flotation element is partially blown sideways and away from air coolers, not onto the flotation deck. An automatic orientation by acting as a weathercock deviating from direct headwind, as described here can be achieved by arranging the turret on one side of the longitudinal axis of the float, and / or by having the superstructure aboard the floating element act as a wind sail to turn the floating element to one side.

[0064] Those skilled in the art will understand the details and variations for the anchored turret, the storage arrangement for the float to align with the wind, and the turret which enables gas transfer from the fixed pipeline direction into the variable direction of the float, all of which are shown as element 11 in figure 5.

[0065] The air cooler effect can be described by the following equation:

Q = UA<*>LMTD

where

Q = power, W

UA = air cooler size, W / ° C

LMTD = logarithmically averaged temperature difference (between air and process fluid in air coolers)

[0066] As an example, air coolers with and UA of 1.4e + 6 W/°C would occupy a footprint area of 300m<2>. If the LMTD is 30 °C, the total cooling power will be 42 MW. If the LMTD for these coolers is reduced to 15 °C, the power or capacity for cooling process liquids is reduced to 21 MW. But in this case, the process fluid or cooling temperature will be much closer to the air temperature, i.e. colder. In most cases, this will significantly improve process efficiency. The effect of air coolers is reduced by reducing the LMTD, but can be increased according to the invention by means of the channel with free space both upwards and downwards, double the number of air coolers. Then the power is brought back up to 42 MW, while simultaneously maintaining the LMTD of 15 °C, and the corresponding colder coolant temperatures.

[0067] Another example shows the operating conditions of the duct for a particular air cooling effect. Starting from a float 350 m long, with a 300 m rectangular channel 15 m wide and 12 m high. The total area for air coolers, assuming sufficient space for access and maintenance, is 3000 m<2>for air coolers facing upwards, and the same for air coolers facing downwards. The total air cooler area is 6000 m<2.> This gives a total UA of 1.4 E + 6 x (6000/300) W / ° C, or 28 MW / ° C. Furthermore, the LMTD is 22 ° C, which gives a total cooling effect of 28 x 22 MW or 616 MW.

[0068] For the e7 baseload LNG system with a capacity of 400 tons of LNG per hour, the cooling power is 236.9 MW according to Table 1. With a cooling capacity of 616 MW, the production capacity is 400 x (616 / 236.9) metric tons per hour, or around 1040 tonnes of LNG per hour.

[0069] Table 2 gives an overview of the duct and the air coolers' operating conditions for this example. The mass flow of air is 12,300 kg/s and the air velocity at the inlet duct is 57 m/s. This velocity is gradually reduced to near zero at the rear duct outlet due to the air consumption of the air coolers. The total pressure drop in the channel inlet and the channel itself is approx. 0.006 bar. An 8.5 MW fan is required to overcome this pressure drop.

[0070] A highly efficient air duct intake filter has been assumed, similar to filters used in gas turbines in coastal areas. A less efficient filter or an enlarged air intake will reduce the pressure drop and reduce the fan power requirement, for example to 2 or 3 MW. The amount of air is large, but can be reduced by increasing the air temperature increase in the air coolers. This will reduce the air speed in the duct, and further reduce the fan power.

[0071] A person skilled in the field will know that increasing the air temperature in air coolers can be done by reducing the number of stages in a train of compressors and intercoolers in the cooling system, such as from three to two stages, while maintaining the total pressure increase. This will significantly increase the outlet temperature from the remaining compressor stage(s), feeding air coolers with much hotter process fluid, which can therefore heat the air to a higher temperature. As an illustration, instead of heating the air from 20 ° to 80 ° C, it can be heated from 20 ° to 140 ° C, which reduces the air flow by about 50%. Compressors that can work with fewer stages and a higher pressure increase over each stage can, for example, use supersonic shock wave technology instead of conventional turbo-compressor technology

[0072] A third example shows how the compressor effect is reduced by increasing the available area for air cooling, for example in a channel, where air from the air coolers can be released vertically downwards in addition to vertically upwards. The compressor can be an integral part of a natural gas condensation process or a general gas compression process.

[0073] Assume a compressor system comprising a first-stage compressor, an air-cooled intercooler, a second-stage compressor, and an air-cooled aftercooler. Methane is compressed from 2 to 6 bara in the first stage, and 6 to 11.5 bara in the second stage. The methane flow rate is 1.0 e + 6 kg/hour. The pressure drop in the air coolers is minimal and therefore ignored in this example.

[0074] The compressor stage has an effect of 69.8 MW in all cases. Air cooler 1 has a UA of 1.4 E + 6 W / ° C when the available space is 300 m2. As footprint increases, UA and airflow increase proportionally. The result of this is that the methane cools more, to a temperature that is closer to the inlet air temperature. In case 1, the methane temperature is 60 °C after the intercooler. In case 2, which has 50% more air cooling capacity, the methane temperature is 40°C out of the intercooler. In case 3, with twice the cooling area available compared to case 1, the methane temperature is lowered to 29 °C on the downstream side of the intercooler.

[0075] The colder the methane that flows to compressor stage 2 is, the lower the volume, and the power of compressor 2 therefore decreases from 44.3 to 40.2 MW, or by about 10%, when the air cooling capacity is increased by a factor of two .

[0076] The compressor aftercooler capacity is also increased from case 1 to case 3. There is a lower methane inlet temperature, 126.6 °C in case one, 104.4 °C in case two, and 92.0 °C in case three with the largest intercooler. The result of this, as well as increased aftercooler capacity, is a significantly reduced aftercooler outlet temperature, which starts at 60.4°C in case 1, 35.7°C in case 2 and 25.9°C in case 3. If the compressed and cooled methane is used in a refrigeration plant, colder gas from case 3 will be far more efficient.

[0077] A fourth example is a compressor aftercooler, cooled with water that circulates between the aftercooler and the air cooler. This indirect cooling of the compressed gas can simplify the system and improve its safety. This example shows that indirect cooling is more efficient, and the aftercooler becomes much smaller, when the air's cooling capacity is increased according to the invention.

[0078] Consider a compressor, which compresses 1.0 E + 6 kg / hour of methane from 2 to 6 bara. The aftercooler is a methane/water heat exchanger. Cold water flows into the aftercooler, where it is heated. The hot water is then pumped to an air cooler, which removes the same amount of energy that is supplied in the methane/water aftercooler. The results are shown in table 4.

[0079] The compressor operation is the same for cases 1, 2 and 3. The cooling of methane in the aftercooler is also the same, although the advantage of the larger air cooler may have been used to cool the methane to a lower temperature.

[0080] The advantage of larger air coolers in this example is, as Table 4 shows, to provide colder cooling water to the methane/water heat exchanger. This means that, for the same service, the LMDT of the water/methane heat exchanger is increased from 7.5 °C in case 1, to 28.9 °C in case 2 and further to 36.4 °C in case 3. The heat exchanger UA , or size, is correspondingly reduced. This saves space and costs in the compressor aftercooler-heat exchanger. The air cooler footprint, made possible according to the present invention, increases the air flow and the air cooler UA proportionally from case 1 to cases 2 and 3. As a result of the size of the process equipment, in this case the water/methane heat exchanger is reduced and frees up valuable space on the floater's deck .

[0081] A fifth example is a steam cycle, which is used to power many floaters. This example shows that the steam cycle becomes more efficient, and output power increases, when the air cooling capacity is increased according to the invention. Low pressure steam from a steam turbine flows to an air cooler where the steam condenses. The condensate is pumped to a heat source, typically a boiler, where it is evaporated and super-heated. The high-pressure steam drives the steam turbine.

[0082] The results are shown in table 5.1 similar to examples three and four, the air cooler footprint is increased from 300 to 450 m2 and 600 m2 for cases 1, 2 and 3 respectively.

[0083] Heat output and pressure and temperature from the boiler are the same in each individual case. The steam conditions will therefore not affect the system's efficiency. For the expander or steam turbine, outlet pressure decreases when the air cooling capacity is increased from case 1 to cases 2 and 3. As a result, output power increases from 6.18 to 6.64 and 6.86 MW when the allowable air cooler footprint is increased by 50 and 100 respectively %. This is an 11% increase in power consumption. There is no significant effect on the condensate pump, with the exception of a small reduction in the flow rate of the condensate in cases 2 and 3.

[0084] For a professional in the field, and depending on permits and environmental conditions, it will be possible to optimize the system by partial use of sea water for cooling, for example by means of a submerged pipe in which hot water is introduced, flows and is cooled by a heating line to the surrounding seawater, is released back and returned to the process for reuse as coolant, after different distribution of NGL fractionation tasks between the terminal and CLSO, and using alternative liquefaction processes such as N2 coolant for smaller systems. In addition, the float turret and associated swivel can be located in the bow or on the foredeck; the available space in the cantilever and / or channel, freed up by mounting some air coolers for vertically downward directed emission of air, can be used for other equipment; the channel can have a rectangular, rhombic or other shape; air cooling capacity can be increased by mounting some coolers elsewhere on the float; a distance can be provided from the air duct to the first air coolers to further prevent air recirculation; and the sequence of appliances in the duct inlet can be modified, for example by using chillers to cool the air instead of water spray.

Claims (11)

1. An air cooler assembly comprising a number of air coolers (1), characterized in that the air coolers (1) are arranged on a channel (15), that the channel has the shape of a straight prism which has a polygonal cross-section, and where the channel has one air intake (14 ) to take in cooling air for distribution to all air coolers (1). 2. The air cooler assembly according to claim 1, characterized in that the longitudinal axis of the channel is essentially arranged horizontally. 3. The air cooler arrangement according to claim 1 or 2, characterized in that the air intake (14) is provided with one or more fans (e) (16). 4. The air cooler assembly as stated in any of the preceding claims, characterized in that the air inlet (14) is provided with separators (18) for separating liquid and particles from the incoming air. 5. The air cooler assembly as stated in any of the preceding claims, characterized in that the air inlet (14) is provided with one or more spray nozzle(s) (19) for moistening and cooling incoming air. 6. The air cooler assembly according to claim 5, characterized in that a filter (20) for removing small water droplets is arranged downstream of the spray nozzle(s) (19). 7. A floater (10) for LNG production comprising a plurality of air coolers (1) to achieve the cooling capacity required, characterized in that the air coolers (1) are arranged on a channel (15), that the channel has the shape of a straight prism with a polygonal cross-section, and where the channel has one air intake (14) to take in cooling air to be distributed to all the air coolers (1). 8. The float according to claim 7, characterized in that the float (10) is anchored by means of a turret and swivel (11), where the float is allowed to turn according to the wind to keep the air inlet (14) against the wind. 9. The float according to claim 7 or 8, characterized in that the float is an elongated ship-shaped float which has a bow end and an aft end, where the turret and swivel (11) are arranged at the bow end of the float, and where the channel is arranged essentially parallel with the longitudinal axis of the float. 10. Procedure for producing LNG from natural gas on board a float, characterized by the following steps: • bringing pre-treated natural gas on board a float, • cooling the natural gas to produce LNG by repeated cycles consisting of compression, cooling and expansion of a refrigerant and heat exchange between the cold refrigerant and the natural gas to cool the natural gas, characterized in that cooling of the refrigerant during the production of LNG is carried out by means of air coolers which are arranged on one or more channels which are arranged so that all air coolers receive air from the inside of the said one or more channel(s), and where the air which used in air coolers is released to the environment through the air cooler (e). 11. The method according to claim 10, characterized in that the channel (e) (is) oriented so that the air inlet thereof is against the wind in relation to the air coolers arranged on the pipe.